Fluorine-containing electrolyte membranes are mainly used in solid polymer fuel cells currently attracting attention. However, they have problems. For improving the power generating characteristics of fuel cells, such means is available as increasing the ion exchange capacity of the electrolyte membrane or diminishing the membrane thickness, for instance. In either case, however, decreases in mechanical strength will surely result. Since the membrane is firmly compressed during use, the membrane undergoes deformation and degradation due to the creep phenomenon, or is deteriorated due to repeated expansion and shrinkage on the occasions of starting and stopping power generation and, in extreme cases, pinholes are made, allowing mixing of hydrogen and oxygen with each other.
For preventing fluorine-containing electrolyte membranes from being deteriorated, it has been proposed that the electrolyte membranes be crosslinked and thus converted to cured membranes (cf. e.g. Patent Document 1: Japanese Kokai Publication S60-133031; Patent Document 2: Japanese Kokai Publication S54-107889; Patent Document 3: Japanese Kokai Publication S54-52690; Patent Document 4: Japanese Kokai Publication S61-276828; Patent Document 5: Japanese Kokai Publication 2000-188013; Patent Document 6: Japanese Kokai Publication 2002-53619; Patent Document 7: Japanese Kokai Publication 2003-128833). As for the method of obtaining cured membranes, a method is known which comprises blending a resin with a crosslinking agent and subjecting the mixture to extrusion molding. However, there are problems; the crosslinking reaction already occurs in the molding machine and, therefore, it is difficult to control the crosslinking reaction, hence it is difficult to manufacture membranes constant in quality.
Also known as a method of producing fluorine-containing cured electrolyte membranes is the method comprising blending a resin with a crosslinking agent and subjecting the mixture to hot press molding (cf. e.g. Patent Document 6). However, this method has a problem: it is difficult to obtain membranes large in size, so that batch production is unavoidable and mass production is difficult to make.
Another known method of producing fluorine-containing cured electrolyte membranes comprises impregnating perfluoro type sulfonyl fluoride membranes made in the conventional manner with a crosslinking agent and curing them by heating or exposure to high energy radiation (cf. e.g. Patent Document 5). However, this document does not disclose the crosslinking of cast membranes formed from a solution, following by heating, for instance. Further, the crosslinking agent is generally a large molecule and it is difficult for that agent to uniformly penetrate into the membranes; it is a problem that uniformly cured membranes are thus difficult to obtain.
A further known method of producing fluorine-containing cured electrolyte membranes comprises blending a ˜SO2F type dispersion with a crosslinking agent and, after membrane formation by casting, curing the membranes by heating (cf. e.g. Patent Document 7). However, the ˜SO2F type dispersion contains the emulsifier and/or initiator residue, which produces a problem, namely deteriorates the characteristics of the membranes obtained.